Laser device, laser module, semiconductor laser and fabrication method of semiconductor laser

ABSTRACT

A semiconductor laser has first and second diffractive grating regions. The first diffractive grating region has segments, has a gain, and has first discrete peaks of a reflection spectrum. The second diffractive grating region has segments combined to each other, and has second discrete peaks of a reflection spectrum. Each segment has a diffractive grating and a space region. Pitches of the diffractive grating are substantially equal to each other. A wavelength interval of the second discrete peaks is different from that of the first discrete peaks. A part of a given peak of the first discrete peaks is overlapped with that of the second discrete peaks when a relationship between the given peaks of the first discrete peaks and the second discrete peaks changes. A first segment located in the first diffractive grating region or the second diffractive grating region has an optical length shorter or longer than the other segments of the first diffractive grating region and the second diffractive grating region by odd multiple of half of the pitch of the diffractive grating of the first diffractive grating region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of Ser. No. 11/616,530,filed Dec. 27, 2006, which is based upon and claims the benefit ofpriority from the prior Japanese Patent Application No. 2005-376050,filed Dec. 27, 2005 and Japanese Patent Application No. 2006-316887,filed Nov. 24, 2006, the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a laser device, a laser module, asemiconductor laser and a fabrication method of the semiconductor laser.

2. Description of the Related Art

Generally, a wavelength-changeable semiconductor laser has asemiconductor element having a gain for a laser and awavelength-selectable semiconductor element. The semiconductor laserhas, for example, a Sampled Grating Distributed Feedback Laser (SG-DFB)region and a Sampled Grating Distributed Reflector (SG-DR) region.

The semiconductor laser emits a desirable laser light with use of avernier effect. That is, the laser light is emitted at a wavelengthwhere a longitudinal mode spectrum of the SG-DFB region corresponds to areflection spectrum of the SG-DR region, in the semiconductor laser.Therefore, it is possible to emit a desirable laser by controlling thelongitudinal mode spectrum of the SG-DFB region and the reflectionspectrum of the SG-DR region.

In the semiconductor laser, a phase difference of 90 degrees is,however, generated between a light incoming to the SG-DR region and theSG-DFB region and a light reflected by a diffractive grating of theSG-DR region and the SG-DFB region in a design wavelength range of thediffractive grating. Therefore, a phase difference of 180 degrees isgenerated between lights transmitting in a resonator in directionsopposite to each other, in the design wavelength range. Accordingly, adesirable wavelength light is canceled and it is possible that a laseris not emitted.

On the other hand, the phase difference in the diffractive grating isoffset from 90 degrees at a wavelength offset to shorter wavelength orlonger wavelength from the design wavelength of the diffractive grating.And it is possible that a single wavelength laser is not emitted.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor laser that can emit alaser at a desirable wavelength.

According to an aspect of the present invention, preferably, there isprovided a semiconductor laser including a first diffractive gratingregion and a second diffractive grating region. The first diffractivegrating region has a plurality of segments combined to each other, has again, and has first discrete peaks of a reflection spectrum. Each of thesegments has a diffractive grating and a space region. The seconddiffractive grating region is combined to the first diffractive gratingregion, has a plurality of segments combined to each other, and hassecond discrete peaks of a reflection spectrum. Each of the segments hasa diffractive grating and a space region. The pitch of the diffractivegrating is substantially same as that of the diffractive grating of thefirst diffractive grating region. A wavelength interval of the seconddiscrete peaks is different from that of the first discrete peaks. Apart of a given peak of the first discrete peaks is overlapped with apart of a given peak of the second discrete peaks in a case where arelationship between the given peak of the first discrete peaks and thegiven peak of the second discrete peaks changes. A first segment locatedin the first diffractive grating region or the second diffractivegrating region has an optical length shorter or longer than the othersegments of the first diffractive grating region and the seconddiffractive grating region by odd multiple of half of the pitch of thediffractive grating of the first diffractive grating region.

With the above-mentioned configuration, a phase of a light incoming tothe first diffractive grating region changes by 90 degrees, when thediffractive grating reflects the light. A phase of a light incoming tothe second diffractive grating region changes by 90 degrees, when thediffractive grating reflects the light. And, the phase of the lightreflected by the diffractive grating of the first diffractive gratingregion is different from that of the light reflected by the diffractivegrating of the second diffractive grating region by 180 degrees. Oddmultiple of half of the pitch of the diffractive grating of the firstdiffractive grating region corresponds to 90 degrees phase difference.Therefore, a phase of a light changes by 180 degrees, when the lighttravels back and forth in the first segment. Accordingly, a phasedifference between lights is substantially zero, the lights traveling inopposite directions in the first diffractive grating region and thesecond diffractive grating region. And the semiconductor laser emits alaser of a desirable wavelength, when a part of the given peak of thefirst discrete peaks is overlapped with a part of the given peaks of thesecond discrete peaks.

According to another aspect of the present invention, preferably, thereis a fabrication method of a semiconductor laser. The method includescoating a resist on a semiconductor layer, exposing a diffractivegrating pattern to the resist, exposing a pattern having a plurality ofspace regions to the resist and transferring a resist pattern formed bydeveloping the resist to the semiconductor layer, the diffractivegrating pattern having a convex portion or a concave portion of anoptical length that is odd multiple of a pitch of a diffractive grating,the space region separating the diffractive grating pattern.

With the above-mentioned configuration, the resist is coated on thesemiconductor layer. The diffractive grating pattern is exposed to theresist. The pattern corresponding to the space regions is exposed to theresist. The resist pattern is transferred to the semiconductor layer. Inthis case, it is possible to fabricate a semiconductor laser that has adiffractive grating pattern having a convex portion or a concaveportion, the optical length of the convex portion or the concave portionbeing odd multiple of half of the pitch of a diffractive gratingAccording to another aspect of the present invention, preferably, thereis provided a laser module including a semiconductor laser, a reflectionspectrum changeable portion and a terminal. The semiconductor laser hasa first diffractive grating region and a second diffractive gratingregion. The first diffractive grating region has a plurality of segmentscombined to each other, has a gain, and has first discrete peaks of areflection spectrum. The second diffractive grating region is combinedto the first diffractive grating region, has a plurality of segmentscombined to each other, and has second discrete peaks of a reflectionspectrum. Each of the segments has a diffractive grating and a spaceregion. A pitch of the diffractive grating of the second diffractivegrating region is substantially same as that of the first diffractivegrating region. A wavelength interval of the second discrete peaks isdifferent from that of the first discrete peaks. A part of a given peakof the first discrete peaks is overlapped with a part of a given peak ofthe second discrete peaks in a case where a relationship between thegiven peak of the first discrete peaks and the given peak of the seconddiscrete peaks changes. The reflection spectrum changeable portionchanges at least either the first discrete peaks or the second discretepeaks. The terminal is for controlling the reflection spectrumchangeable portion. A first segment located in the first diffractivegrating region or the second diffractive grating region has an opticallength shorter or longer than the other segments of the firstdiffractive grating region and the second diffractive grating region byodd multiple of half of the pitch of the diffractive grating of thefirst diffractive grating region.

With the above-mentioned configuration, a phase of a light incoming tothe first diffractive grating region changes by 90 degrees, when thediffractive grating reflects the light. A phase of a light incoming tothe second diffractive grating region changes by 90 degrees, when thediffractive grating reflects the light. And, the phase of the lightreflected by the diffractive grating of the first diffractive gratingregion is different from that of the light reflected by the diffractivegrating of the second diffractive grating region by 180 degrees. Oddmultiple of half of the pitch of the diffractive grating of the firstdiffractive grating region corresponds to 90 degrees phase difference.Therefore, a phase of a light changes by 180 degrees, when the lighttravels back and forth in the first segment. Accordingly, a phasedifference between lights is substantially zero, the lights traveling inopposite directions in the first diffractive grating region and thesecond diffractive grating region. Further, it is possible to controlthe discrete peaks of the first diffractive grating region and thesecond diffractive grating region, when the reflection spectrumchangeable portion is controlled from outside through the terminal. Andthe semiconductor laser emits a laser of a desirable wavelength.

According to another aspect of the present invention, preferably, thereis provided a laser device including a semiconductor laser, a reflectionspectrum changeable portion and a controller. The semiconductor laserhas a first diffractive grating region and a second diffractive gratingregion. The first diffractive grating region has a plurality of segmentscombined to each other, has a gain, and has first discrete peaks of areflection spectrum. The second diffractive grating region is combinedto the first diffractive grating region, has a plurality of segmentscombined to each other, and has a second discrete peaks of a reflectionspectrum. Each of the segments has a diffractive grating and a spaceregion. A pitch of the diffractive grating of the second diffractivegrating region is substantially same as that of the first diffractivegrating region. A wavelength interval of the second discrete peaks isdifferent from that of the first discrete peaks. A part of a given peakof the first discrete peaks is overlapped with a part of a given peak ofthe second discrete peaks in a case where a relationship between thegiven peak of the first discrete peaks and the given peak of the seconddiscrete peaks changes. The reflection spectrum changeable portionchanges at least either the first discrete peaks or the second discretepeaks. The controller controls the reflection spectrum changeableportion. A first segment located in the first diffractive grating regionor the second diffractive grating region has an optical length shorteror longer than the other segments of the first diffractive gratingregion and the second diffractive grating region by odd multiple of halfof the pitch of the diffractive grating of the first diffractive gratingregion.

With the above-mentioned configuration, a phase of a light incoming tothe first diffractive grating region changes by 90 degrees, when thediffractive grating reflects the light. A phase of a light incoming tothe second diffractive grating region changes by 90 degrees, when thediffractive grating reflects the light. And, the phase of the lightreflected by the diffractive grating of the first diffractive gratingregion is different from that of the light reflected by the diffractivegrating of the second diffractive grating region by 180 degrees. Oddmultiple of half of the pitch of the diffractive grating of the firstdiffractive grating region corresponds to 90 degrees phase difference.Therefore, a phase of a light changes by 180 degrees, when the lighttravels back and forth in the first segment. Accordingly, a phasedifference between lights is substantially zero, the lights traveling inopposite directions in the first diffractive grating region and thesecond diffractive grating region. Further, it is possible to controlthe discrete peaks of the first diffractive grating region and thesecond diffractive grating region, when the controller controls thereflection spectrum changeable portion. And the semiconductor laseremits a laser of a desirable wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail with reference to the following drawings, wherein:

FIG. 1 illustrates a schematic view of a laser device in accordance witha first embodiment of the present invention;

FIG. 2A and FIG. 2B illustrate details of a semiconductor laser;

FIG. 3 illustrates details of each of segments in waveguide cores;

FIG. 4A through FIG. 4D illustrate details of sampled diffractivegrating of a segment;

FIG. 5 illustrates details of a sampled diffractive grating of asegment;

FIG. 6 illustrates details of a sampled diffractive grating of asegment;

FIG. 7A through FIG. 7F illustrate a flow of a fabrication method of asemiconductor laser;

FIG. 8A through FIG. 8C illustrate a flow of a fabrication method of asemiconductor laser;

FIG. 9 illustrates a semiconductor laser in accordance with a secondembodiment of the present invention;

FIG. 10 illustrates details of each of segments in waveguide cores;

FIG. 11 illustrates a semiconductor laser in accordance with a thirdembodiment of the present invention;

FIG. 12A and FIG. 12B illustrate a segment in accordance with a fourthembodiment of the present invention;

FIG. 13A and FIG. 13B illustrate another segment in accordance with thefourth embodiment;

FIG. 14 illustrates a segment in accordance with a fifth embodiment ofthe present invention;

FIG. 15A and FIG. 15B illustrate another segment in accordance with thefifth embodiment;

FIG. 16 illustrates a segment in accordance with a sixth embodiment ofthe present invention; and

FIG. 17 illustrates another segment in accordance with the sixthembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to the accompanyingdrawings, of embodiments of the present invention.

FIG. 1 illustrates a schematic view of a laser device 100 in accordancewith a first embodiment of the present invention. As shown in FIG. 1,the laser device 100 has a laser module 200 and a controller 300. Thelaser module 200 has a semiconductor laser 201, beam splitters 202 and203, a rocking etalon 204, optical detector elements 205 and 206, atemperature control device 207 and terminals 208 through 211. Thesemiconductor laser 201 is provided on the temperature control device207.

The semiconductor laser 201 emits a laser having a given wavelength. Apart of a laser light from the semiconductor laser 201 is reflected bythe beam splitter 202 and is provided to the beam splitter 203. The restof the laser light from the semiconductor laser 201 transmits the beamsplitter 202 and is emitted toward outside. A part of the laser lightfrom the beam splitter 202 is reflected by the beam splitter 203 and isprovided to the optical detector element 206. The rest of the laserlight from the beam splitter 202 transmits the beam splitter 203 and isprovided to the rocking etalon 204. The laser light provided to therocking etalon 204 has wavelength peaks at a given period and isprovided to the optical detector element 205.

The optical detector element 205 measures an optical intensity of alight from the rocking etalon 204. The optical detector element 205converts the measured result into an electrical signal and provides theelectrical signal to the controller 300 through the terminal 211. Theoptical detector element 206 measures an optical intensity of a lightfrom the beam splitter 203. The optical detector element 206 convertsthe measured result into an electrical signal and provides theelectrical signal to the controller 300 through the terminal 210.

The controller 300 has a Central Processing Unit (CPU), a Random AccessMemory (RAM), a Read Only Memory (ROM) and so on. The ROM of thecontroller 300 stores control information and a control program of thesemiconductor laser 201. The controller 300 detects an emissionwavelength of the semiconductor laser 201 according to the measuredresult from the optical detector element 205. The controller 300controls the temperature control device 207 and the semiconductor laser201 according to a ratio of the measured result of the optical detectorelement 205 and the measured result of the optical detector element 206.In this case, the controller 300 provides an electrical signal to thesemiconductor laser 201 through the terminal 209 and provides anelectrical signal to the temperature control device 207 through theterminal 208.

FIG. 2A and FIG. 2B illustrate details of the semiconductor laser 201.FIG. 2A illustrates a top view of the semiconductor laser 201. FIG. 2Billustrates a cross sectional view taken along a line A-A of FIG. 2A. Adescription will be given, with reference to FIG. 2A and FIG. 2B, of thesemiconductor laser 201. As shown in FIG. 2A and FIG. 2B, thesemiconductor laser 201 has a structure in which a Sampled GratingDistributed Reflector (SG-DR) region A and a Sampled Grating DistributedFeedback Laser (SG-DFB) region B are coupled in order.

The SG-DR region A has a structure in which a waveguide core 3, acladding layer 5 and an insulating layer 6 are laminated on a substrate1 in order and thin film resistors 9, power electrodes 10 and a grandelectrode 11 are laminated on the insulating layer 6. The SG-DFB regionB has a structure in which a waveguide core 4, the cladding layer 5, acontact layer 7 and an electrode 8 are laminated on the substrate 1 inorder. The substrate 1 and the cladding layer 5 of the SG-DR region Aand the SG-DFB region B are a single layer formed as a unitrespectively. The waveguide cores 3 and 4 are formed on a same plane andform a waveguide core together.

A low reflecting coating 12 is formed on end faces of the substrate 1,the waveguide core 3 and the cladding layer 5 at the SG-DR region Aside. On the other hand, a low reflecting coating 13 is formed on endfaces of the substrate 1, the waveguide core 4 and the cladding layer 5at the SG-DFB region B side. Sampled diffractive gratings 2 are formedat a given interval in the waveguide cores 3 and 4. The sampled gratingis thus formed.

The substrate 1 is, for example, a semiconductor substrate composed ofInP. The waveguide core 3 is, for example, composed of InGaAsPcrystalline having an absorption edge wavelength at shorter wavelengthsside compared to the laser emission wavelength. PL wavelength of thewaveguide core 3 is approximately 1.3 μm. The waveguide core 4 is, forexample, an active layer composed of InGaAsP crystalline amplifying alight of a desirable wavelength of a laser emission. The PL wavelengthof the waveguide core 4 is approximately 1.57 μm.

The coupling constant of the sampled diffractive grating 2 isapproximately 200 cm⁻¹. The pitch λ of the sampled diffractive grating 2is approximately 0.24 μm. The number of asperity of the sampleddiffractive grating 2 is approximately 17. The refractive index of thesampled diffractive grating 2 is approximately 3.2. The length of thesampled diffractive grating 2 is approximately 4 μm. The braggwavelength of the sampled diffractive grating 2 is approximately 1.55μm. In this case, the reflectivity against the bragg wavelength of thesampled diffractive grating 2 is approximately less than 1%.

Five SG-DR segments are formed in the waveguide core 3. Here, the SG-DRsegment is a region in which one region having the sampled diffractivegrating 2 and one space region not having the sampled diffractivegrating 2 are combined in the waveguide core 3. Each optical length ofthe SG-DR segments is substantially equal to each other. “Substantiallyequal” means that differences between each length of the space regionsare less than 1% of the average length of the space regions.

In general, it is possible to enhance an interference effect of lightreflected by the sampled diffractive grating 2, when the number of theSG-DR segments is increased. And a mode stability of a laser emission isthus improved. However, element length is increased if the number of theSG-DR segments is increased. In addition, the mode stability issaturated because of an internal loss in the waveguide core 3, whentotal length of the waveguide core 3 is over 2 mm. It is, therefore,preferable that the number of the SG-DR segments is approximately 5.Each length of the SG-DR segments is, for example, 270 μm.

Five SG-DFB segments are formed in the waveguide core 4. Here, theSG-DFB segment is a region in which one region having the sampleddiffractive grating 2 and one space region not having the sampleddiffractive grating 2 are combined in the waveguide core 4. And theSG-DFB segment has a gain. The number of the SG-DFB segments ispreferably as same as that of the SG-DR segments, and is, for example,5. Each length of the SG-DFB segments is substantially equal to eachother, and is, for example, 240 μm. “Substantially equal” means thatdifferences between each length of the space regions are approximatelyless than 1% of the average length of the space regions.

The cladding layer 5 is composed of InP, constricts a current andconfines a laser light traveling in the waveguide cores 3 and 4. Thecontact layer 7 is composed of InGaAsP crystalline. The insulating layer6 is a protection film composed of an insulator such as SiN or SiO₂. Thelow reflecting coatings 12 and 13 are, for example, composed of adielectric film including MgF₂ and TiON. The reflectivity of the lowreflecting coatings 12 and 13 is, for example, less than 0.3%.

The thin film resistor 9 is composed of such as NiCr, and controls eachtemperature of the SG-DR segments according to the intensity of acurrent provided from the controller 300 in FIG. 1. Each of the thinfilm resistors 9 is formed on the insulating layer 6 above each of theSG-DR segments respectively. Each of the power electrodes 10 is coupledto each of the thin film resistors 9. The grand electrode 11 is coupledto each of the thin film resistor 9. The power electrodes 10, the grandelectrode 11 and the electrode 8 are composed of a conductive materialsuch as Au.

Next, a description will be given of an operation of the semiconductorlaser 201. At first, a given current is provided to the electrode 8 fromthe controller 300 in FIG. 1. And a light is generated in the waveguidecore 4. The light propagates in the waveguide cores 3 and 4, isreflected and amplified repeatedly, and is emitted toward outside.

The controller 300 can control an emission wavelength of thesemiconductor laser 201. The controller 300 provides a given current tothe temperature control device 207 in FIG. 1. It is thus possible tocontrol a temperature of the semiconductor laser 201 to be a givenvalue. And it is possible to control a local maximum peak reflectivityof the waveguide core 4. Further, the controller 300 provides a givencurrent to each of the thin film resistors 9. In this case, it ispossible to control the temperature of the SG-DR segments separately.The refractive index of each of the SG-DR segments is changed to be agiven value. Therefore, it is possible to control the local maximum peakreflectivity of the waveguide core 3. And the controller 300 can controlthe emission wavelength of the semiconductor laser 201.

FIG. 3 illustrates details of each of the segments in the waveguidecores 3 and 4. As shown in FIG. 3, a space region of a segment 41 has anoptical length longer or shorter than that of segments other than thesegment 41 by odd multiple of half of the pitch λ of the sampleddiffractive grating 2. The segment 41 is one of the SG-DR segments andthe SG-DFB segments. The segments other than the segment 41 have anoptical length that is integral multiple of the pitch λ of the sampleddiffractive grating 2. In the embodiment, the optical lengths of thesegments other than the segment 41 are substantially equal to eachother, and the optical length of the space region of the segment 41 islonger than the other segments by half of the pitch λ of the sampleddiffractive grating 2.

In this case, a phase of a light incoming to each of the SG-DR segmentschanges by 90 degrees, when the sampled diffractive grating 2 reflectsthe light. On the other hand, a phase of a light incoming to each of theSG-DFB segments changes by 90 degrees, when the sampled diffractivegrating 2 reflects the light. And, the phase of the light reflected bythe sampled diffractive grating 2 of the SG-DR segment is different fromthat of the light reflected by the sampled diffractive grating 2 of theSG-DFB segment by 180 degrees. Odd multiple of half of the pitch λcorresponds to 90 degrees phase difference. Therefore, a phase of alight changes by 180 degrees, when the light travels back and forth inthe segment 41. Accordingly, a phase difference between lights issubstantially zero, the lights traveling in opposite directions in thewaveguide cores 3 and 4. And the semiconductor laser 201 can emit alaser of a desirable wavelength.

The optical length of the sampled diffractive grating 2 of the segment41 may be longer or shorter than the sampled diffractive grating 2 ofother segments by odd multiple of half of the pitch λ of the sampleddiffractive grating 2. In this case, the optical length of the spaceregion of the segment 41 is substantially same as that of othersegments. FIG. 4A through FIG. 4D illustrate details of the sampleddiffractive grating 2 of the segment 41.

In FIG. 4A, one of concave portions of the sampled diffractive grating 2of the segment 41 has an optical length longer than the other concaveportions by half of the pitch λ. In FIG. 4B, one of convex portions ofthe sampled diffractive grating 2 of the segment 41 has an opticallength longer than the other convex portions by half of the pitch λ. InFIG. 4C, two of the concave portions of the sampled diffractive grating2 of the segment 41 have an optical length longer than the other concaveportions by one-fourth of the pitch λ. In FIG. 4D, two of the convexportions of the sampled diffractive grating 2 of the segment 41 have anoptical length longer than the other convex portions by one-fourth ofthe pitch λ.

In cases of FIG. 4A through FIG. 4D, a phase of a light changes by 180degrees, when the light travels back and forth in the segment 41.Therefore, a phase difference between lights is substantially zero, thelights traveling in opposite directions in the waveguide cores 3 and 4.And the semiconductor laser 201 can emit a laser of a desirablewavelength.

The optical length difference between the segment 41 mentioned above andthe other segments may be distributed into the space region of thesegment 41 and the sampled diffractive grating 2. FIG. 5 illustrates thedetails. As shown in FIG. 5, one of the concave portions and the convexportions of the sampled diffractive grating 2 of the segment 41 has anoptical length longer than the other concave portions and the otherconvex portions by one-fourth of the pitch λ. And the space region ofthe segment 41 has an optical length longer than the other concaveportions and the other convex portions by one-fourth of the pitch λ. Inthis case, a phase of a light changes by 180 degrees, when the lighttravels back and forth in the segment 41. Therefore, a phase differencebetween lights is substantially zero, the lights traveling in oppositedirections in the waveguide cores 3 and 4. And the semiconductor laser201 can emit a laser of a desirable wavelength.

The space region of the segment 41 may have an optical length shorterthan that of the other segments by odd multiple of half of the pitch λof the sampled diffractive grating 2. FIG. 6 illustrates the details. Asshown in FIG. 6, the space region of the segment 41 has an opticallength shorter than that of the other segments by half of the pitch λ ofthe sampled diffractive grating 2. The segments other than the segment41 have an optical length that is odd multiple of the pitch λ of thesampled diffractive grating 2. In the embodiment, the optical lengths ofthe space region of the segments other than the segment 41 aresubstantially equal to each other.

In this case, a phase of a light changes by 180 degrees, when the lighttravels back and forth in the segment 41. Therefore, a phase differencebetween lights is substantially zero, the lights traveling in oppositedirections in the waveguide cores 3 and 4. And the semiconductor laser201 can emit a laser of a desirable wavelength.

It is preferable that the segment 41 mentioned above is one of the twosegments of the SG-DR segments and or one of the two segments of theSG-DFB segments that are on the boundary side between the SG-DR region Aand the SG-DFB region B. This is because a mirror loss closes tosymmetrical at a position between the SG-DR region A and the SG-DFBregion B.

Next, a description will be given of a fabrication method of thesemiconductor laser 201. At first, a description will be given of afabrication method of the semiconductor laser 201 in which the sampleddiffractive grating 2 has a different optical length. FIG. 7A throughFIG. 7F illustrate a flow of the fabrication method of the semiconductorlaser 201. As shown in FIG. 7A, a resist 51 is coated on the substrate1. Next, as shown in FIG. 7B, a transparent mask 52 such as glass isprovided on the resist 51 and the resist 51 is subjected to aninterference exposure. The transparent mask 52 has a step 53 formed onthe resist 51 side thereof. And a light path length in the transparentmask 52 is changed at the step 53. The step 53 is formed so that aninterference fringe changes by half of the pitch λ. And, as shown inFIG. 7C, a part of the resist 51 under the step 53 is not exposed andthe rest of the resist 51 is exposed stripedly, the part of the resist51 being to be a concave portion or a convex portion having an opticallength longer than the other concave portions and the other convexportions by half of the pitch λ.

Next, as shown in FIG. 7D, the resist 51 is covered with an exposuremask 54 having an opening corresponding to a part to be the spaceregion, and is exposed. The part corresponding to the space regionseparating each of the sampled diffractive gratings 2 is exposed. Next,as shown in FIG. 7E, a resist pattern 55 is formed after the part of theresist 51 not exposed is eliminated through an etching treatment. Next,as shown in FIG. 7F, the resist pattern 55 is transferred to thesubstrate 1 through an etching treatment. Therefore, the sampleddiffractive grating 2 and the space region are formed. After that, otherlayers are formed on the substrate 1. And the semiconductor laser 201 isfabricated.

Next, a description will be given of a fabrication method of thesemiconductor laser 201 in which the space region has a differentoptical length. At first, the steps shown in FIG. 7A through FIG. 7C areprocessed. Next, as shown in FIG. 8A, the resist 51 is covered with anexposure mask 56. The exposure mask 56 has an opening corresponding tothe part to be the space region that has a concave portion or a convexportion having an optical length longer than the other concave portionsand the other convex portions by half of the pitch λ, and is exposed.Accordingly, the part corresponding to the space region separating eachof the sampled diffractive gratings 2 is exposed.

Next, as shown in FIG. 8B, a resist pattern 57 is formed after the partof the resist 51 not exposed is eliminated through an etching treatment.Next, as shown in FIG. 8C, the resist pattern 57 is transferred to thesubstrate 1 through an etching treatment. Therefore, the sampleddiffractive grating 2 and the space region are formed. After that, otherlayers are formed on the substrate 1. And the semiconductor laser 201 isfabricated.

In the embodiment, the sampled diffractive grating 2 corresponds to thediffractive grating. The SG-DFB region B corresponds to the firstdiffractive grating region. The SG-DR region A corresponds to the seconddiffractive grating region. The thin film resistor 9 and the temperaturecontrol device 207 correspond to the reflection spectrum changeableportion. The segment 41 corresponds to the first segment. Thetransparent mask 52 corresponds to the phase-shifting mask. Thesubstrate 1 corresponds to the semiconductor layer. The terminal 209corresponds to the terminal for controlling the reflection spectrumchangeable portion. The terminal 208 corresponds to the terminal forcontrolling the temperature control device.

(Second Embodiment)

FIG. 9 illustrates a semiconductor laser 201 a in accordance with asecond embodiment of the present invention. The semiconductor laser 201a has a Chirped Sampled Grating Distributed Reflector (CSG-DR) region Cinstead of the SG-DR region A, being different from the semiconductorlaser 201 in FIG. 2A and FIG. 2B.

The CSG-DR region C has a waveguide core 14 instead of the waveguidecore 3, being different from the SG-DR region A. The waveguide core 14is, for example, composed of InGaAsP crystalline having an absorptionedge wavelength at shorter wavelengths side compared to the laseremission wavelength. PL wavelength of the waveguide core 14 isapproximately 1.3 μm. The waveguides cores 4 and 14 are formed on a sameplane and form a waveguide core together. A plurality of the sampleddiffractive grating 2 is formed at a given interval in the waveguidecore 14. The sampled grating is thus formed. Five CSG-DR segments areformed in the waveguide core 14.

Here, the CSG-DR segment is an region in which one region having thesampled diffractive grating 2 and one space region not having thesampled diffractive grating 2 are combined in the waveguide core 14. Atleast two of the space regions of the CSG-DR segments have an opticallength different from each other.

FIG. 10 illustrates details of each of the segments in the waveguidecores 14 and 4. As shown in FIG. 10, a segment 41 has an optical lengthlonger or shorter than that of segments other than the segment 41 by oddmultiple of half of the pitch λ of the sampled diffractive grating 2.The segment 41 is one of the CSG-DR segments and the SG-DFB segments.The segments other than the segment 41 have an optical length that isintegral multiple longer or odd multiple of half of the pitch λ. Atleast two of the space regions of the CSG-DR segments have an opticallength different from each other.

In the embodiment, the lengths of the CSG-DR segments are, for example,260 μm, 265 μm, 270 μm, 275 μm and 280 μm in order from the SG-DFBregion B side. The optical length of each of the CSG-DR segments changesdependent on the length of the space region.

In the semiconductor laser 201 a, a peak reflectivity of the waveguidecore 14 indicates a local maximum at a given wavelength. It is becausethe phase of the light traveling back and forth in the CSG-DR segmentsin the waveguide core 14 is a value of integral multiplication of 2π atthe given wavelength. On the other hand, the peak reflectivity of thewaveguide core 14 is reduced when the wavelength of the peakreflectivity is as far from the given wavelength. It is becausephase-matched superposition does not occur, as the optical longitudinalmode spacings of the segments are slightly different from each other.

The peak reflection intensity of the optical longitudinal mode of thewaveguide core 14 has wavelength dependence in the semiconductor laser201 a in accordance with the embodiment. That is, the peak reflectionintensity of the optical longitudinal mode of the waveguide core 14changes according to the wavelength. On the contrary, the peakreflection intensity of the waveguide core 4 does not have wavelengthdependence. It is possible to restrain a laser emission within awavelength range in which the peak reflection intensity of the opticallongitudinal mode of the waveguide core 14 is relatively low, and toobtain a stabilized laser emission within a wavelength range in whichthe peak reflection intensity of the optical longitudinal mode of thewaveguide core 14 is relatively high. In addition, it is possible tocontrol the laser emission wavelength easily, when the wavelength rangein which the peak reflection intensity of the optical longitudinal modeis relatively high is changed with the change of the refractive-index ofthe waveguide core 14.

In the embodiment, the CSG-DR region corresponds to the seconddiffractive grating region.

(Third Embodiment)

FIG. 11 illustrates a semiconductor laser 201 b in accordance with athird embodiment of the present invention. The semiconductor laser 201 bfurther has a Power Control (PC) region D, being different from thesemiconductor laser 201 in FIG. 2A and FIG. 2B. The semiconductor laser201 b has a structure in which the SG-DR region A, the SG-DFB region Band the PC region D are coupled in order.

The PC region D has a structure in which a waveguide core 15, thecladding layer 5, a contact layer 16 and an electrode 17 are laminatedon the substrate 1 in order. In the embodiment, the low reflectingcoating 13 is formed on end faces of the substrate 1, the waveguide core15 and the cladding layer 5 at the PC region D side. The substrate 1 andthe cladding layer 5 of the SG-DR region A, the SG-DFB region B and thePC region D are a single layer formed as a unit respectively. Thewaveguides cores 3, 4 and 15 are formed on a same plane and form awaveguide core together. The insulating layer 6 is also formed betweenthe electrode 8 and the electrode 17. The waveguide core 15 is, forexample, composed of InGaAsP crystalline changing an intensity of anoutputting light by absorbing or amplifying a light. PL wavelength ofthe waveguide core 15 is approximately 1.57 μm.

Next, a description will be given of an operation of the semiconductorlaser 201 b. At first, a given current is provided to the electrode 8from the controller 300 in FIG. 2A and FIG. 2B. And a light is generatedin the waveguide core 4. The light is amplified or absorbed, propagatesin the waveguide cores 3 and 4, is reflected and amplified repeatedly,and is emitted toward outside. A given current is provided to theelectrode 17 from the controller 300. And the output of the emittinglight is kept substantially constant. The effect of the presentinvention is obtained, even if the CSG-DR region C is provided insteadof the SG-DR region A.

In the embodiment, the PC region corresponds to the optical absorber orthe optical amplifier.

In the embodiments mentioned above, the optical length difference isgenerated when one of the segments has a physical length different fromthat of the other segments. In the embodiments below, the optical lengthdifference is generated, when a physical quantity other than thephysical length is changed. A description will be give of the details.

(Fourth Embodiment)

Next, a description will be given of a semiconductor laser 201 c inaccordance with a fourth embodiment of the present invention. Thesemiconductor laser 201 c has a segment 41 c instead of the segment 41,being different from the semiconductor laser 201 shown in FIG. 2Athrough FIG. 3. The segment 41 c has a cross section different from thatof the other segments of the waveguide cores 3 and 4. FIG. 12A and FIG.12B illustrate the details.

FIG. 12A illustrates a top view of each segment in the waveguide cores 3and 4 viewing from upward. FIG. 12B illustrates a cross sectional viewcorresponding to FIG. 12A. In the embodiment, the segment 41 c isprovided in the SG-DFB region B. As shown in FIG. 12A, the space regionof the segment 41 c has a small width portion 43. The width of the smallwidth portion 43 is smaller than a region other than the small widthportion 43 (hereinafter referred to a region 44) in the segment 41 c. Inthis case, an equivalent refractive index difference is generatedbetween the segment 41 c and the other segments. Therefore, an opticallength difference is generated between the segment 41 c and the othersegments.

Here, the equivalent refractive index of the small width portion 43 isrepresented as n₁. The equivalent refractive index of the region 44 isrepresented as n₂. The length of the small width portion 43 in thepropagating direction of a light is represented as L. In this case, aphase of a light propagating in the segment 41 c is changed when thelight propagates in the small width portion 43. The phase difference inthis case is represented as Δφ. And the Δφ is shown as followingExpression 1.Δφ=2πn ₁ ×L/λ−2πn ₂ ×L/λ  (Expression 1)

In the embodiment, Δφ is set so that the optical length differencebetween the segment 41 c and the other segments is odd multiple of halfof the pitch λ of the sampled diffractive grating 2. For example, Δφ isset to be −π/2. In this case, a phase of a light changes by 180 degrees,when the light travels back and forth in the segment 41 c. Therefore, aphase difference between lights is substantially zero as in the case ofthe first embodiment, the lights traveling in opposite directions in thewaveguide cores 3 and 4. And the semiconductor laser 201 c can emit alaser of a desirable wavelength.

For example, the equivalent refractive index n₁ is 3.19922 and theequivalent refractive index n₂ is 3.19956, when the width of the smallwidth portion 43 is 1.5 μm, the width of the region 44 is 1.8 μm, andthe thickness of the segment 41 c is adjusted adequately. In this case,Δφ is −π/2 when the length L of the small width portion 43 is 100 μm.And in the segment 41 c, the length of the space region may be 255 μm to277 μm, the length of the sampled diffractive grating 2 may be 3 μm to 5μm, and the length of the segment 41 c may be 260 μm to 280 μm.

The small width portion 43 may be divided into several parts in thesegment 41 c. The segment 41 c may be provided in the SG-DR region A orin the SG-DFB region B. It is preferable that the segment 41 c isprovided at a position closer to the boundary between the SG-DR region Aand the SG-DFB region B. The segment 41 c may have a small thicknessportion instead of the small width portion 43. In this case, it ispossible to control the equivalent refractive index by controlling thethickness of the small thickness portion.

As shown in FIG. 13B, a large width portion 45 having a width largerthan that of the region 44 may be provided instead of the small widthportion 43, although the phase difference is generated because the smallwidth portion 43 is provided in the segment 41 c in the embodiment. Inthis case, the effect of the present invention is obtained when theequivalent refractive indexes of the region 44 and the large widthportion 45 are controlled so that the Δφ is −π/2. That is, the effect ofthe present invention is obtained when the segment 41 c has a regionhaving cross sectional area different from that of the other regionthereof. All of the segments except for one of the SG-DR region A andthe SG-DFB region B may be the segment 41 c. In this case, a phase of alight changes by 180 degrees, when the light travels back and forth inthe segment 41 c.

In the embodiment, the segment 41 c corresponds to the first segment.

(Fifth Embodiment)

Next, a description will be given of a semiconductor laser 201 d inaccordance with a fifth embodiment of the present invention. Thesemiconductor laser 201 d has a segment 41 d instead of the segment 41,being different from the semiconductor laser 201 shown in FIG. 2Athrough FIG. 3. The segment 41 d has a convex portion projectingdownward from a bottom of the space region thereof. The details areshown in FIG. 14.

FIG. 14 illustrates a cross sectional view of each segment of thewaveguide cores 3 and 4. In the embodiment, the segment 41 d is providedin the SG-DFB region B. As shown in FIG. 14, the space region of thesegment 41 d has a convex portion 46 projecting toward the substrate 1.In this case, an equivalent refractive index difference is generatedbetween the segment 41 d and the other segments. Therefore, an opticallength difference is generated between the segment 41 d and the othersegments.

In the embodiment, Δφ is set so that the optical length differencebetween the segment 41 d and the other segments is odd multiple of halfof the pitch λ of the sampled diffractive grating 2. For example, Δφ isset to be −π/2. In this case, a phase of a light changes by 180 degrees,when the light travels back and forth in the segment 41 d. Therefore, aphase difference between lights is substantially zero as in the case ofthe first embodiment, the lights traveling in opposite directions in thewaveguide cores 3 and 4. And the semiconductor laser 201 d can emit alaser of a desirable wavelength.

For example, the equivalent refractive index n₁ of the convex portion 46is 3.19922 when the height of the convex portion 46 is set to be 0.1 μm.An equivalent refractive index of a region other than the convex portion46 in the segment 41 d is set to be 3.18144. In this case, the Δφ is−π/2 when the height of the convex portion 46 is 20 μm, followingExpression 1. And in the segment 41 d, the length of the space regionmay be 255 μm to 277 μm, the length of the sampled diffractive grating 2may be 3 μm to 5 μm, and the length of the segment 41 d may be 260 μm to280 μm.

As shown in FIG. 15A, the convex portion 46 may be divided into severalparts in the segment 41 d. The segment 41 d may be provided in the SG-DRregion A or in the SG-DFB region B. It is preferable that the segment 41d is provided at a position closer to the boundary between the SG-DRregion A and the SG-DFB region B.

The space region of the segment 41 d may have a concave portion 47instead of the convex portion 46 at the bottom thereof. In this case,the substrate 1 may project toward the concave portion 47. In thestructure, the effect of the present invention is obtained, when theequivalent refractive index of the concave portion 47 is adjusted sothat the Δφ is −π/2. That is, the effect of the present invention isobtained when the segment 41 d has a convex portion or a concave portionon the bottom thereof. And, all of the segments except for one of theSG-DR region A and the SG-DFB region B may be the segment 41 d. In thiscase, a phase of a light changes by 180 degrees, when the light travelsback and forth in the segment 41 d.

In the embodiment, the segment 41 d corresponds to the first segment.

(Sixth Embodiment)

Next, a description will be given of a semiconductor laser 201 e inaccordance with a sixth embodiment of the present invention. Thesemiconductor laser 201 e has a segment 41 e instead of the segment 41,being different from the semiconductor laser 201 shown in FIG. 2Athrough FIG. 3. FIG. 16 illustrates a cross sectional view of eachsegment of the waveguide cores 3 and 4. In the embodiment, the segment41 e is provided in the SG-DFB region B.

As shown in FIG. 16, the segment 41 e has a high-refractive-indexportion 48. The high-refractive-index portion 48 has a compositiondifferent from the other region in the segment 41 e (hereinafterreferred to a region 49). And the refractive index of thehigh-refractive-index portion 48 is higher than that of the region 49.In this case, an equivalent refractive index difference is generatedbetween the segment 41 e and the other segments. Therefore, an opticallength difference is generated between the segment 41 e and the othersegments.

In the embodiment, Δφ is set so that the optical length differencebetween the segment 41 e and the other segments is odd multiple of halfof the pitch λ of the sampled diffractive grating 2. For example, Δφ isset to be −π/2. In this case, a phase of a light changes by 180 degrees,when the light travels back and forth in the segment 41 e. Therefore, aphase difference between lights is substantially zero as in the case ofthe first embodiment, the lights traveling in opposite directions in thewaveguide cores 3 and 4. And the semiconductor laser 201 e can emit alaser of a desirable wavelength.

For example, the equivalent refractive index of thehigh-refractive-index portion 48 is 3.20518 when the composition of thehigh-refractive-index portion 48 is adjusted adequately. An equivalentrefractive index of the region 49 is set to be 3.19922, when thecomposition of the region 49 is adjusted adequately. In this case, theΔφ is −π/2 when the length of the high-refractive-portion 48 is 65 μm,following Expression 1. And in the segment 41 e, the length of the spaceregion may be 255 μm to 277 μm, the length of the sampled diffractivegrating 2 may be 3 μm to 5 μm, and the length of the segment 41 e may be260 μm to 280 μm.

The high-refractive-index portion 48 may be provided in the sampleddiffractive grating 2. As shown in FIG. 17A, the high-refractive-indexportion 48 may be divided into several parts in the segment 41 e. Thesegment 41 e may be provided in the SG-DR region A or in the SG-DFBregion B. It is preferable that the segment 41 e is provided at aposition closer to the boundary between the SG-DR region A and theSG-DFB region B. The segment 41 e may have a low-refractive-index regioninstead of the high-refractive-index portion 48, thelow-refractive-index portion having a refractive index lower than thatof the region 49. In this case, the effect of the present invention isobtained, when the difference between the refractive index of thelow-refractive-index portion and that of the region 49 is adjusted sothat the Δφ is −π/2. And, all of the segments except for one of theSG-DR region A and the SG-DFB region B may be the segment 41 e. In thiscase, a phase of a light changes by 180 degrees, when the light travelsback and forth in the segment 41 e.

In the embodiment, the segment 41 e corresponds to the first segment.

While the above description constitutes the preferred embodiments of thepresent invention, it will be appreciated that the invention issusceptible of modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims.

The present invention is based on Japanese Patent Application No.2005-376050 filed on Dec. 27, 2005 and Japanese Patent Application No.2006-316887 filed on Nov. 24, 2006, the entire disclosure of which ishereby incorporated by reference.

1. A fabrication method of a semiconductor laser comprising: coating aresist on a semiconductor layer; exposing a plurality of diffractivegrating patterns to the resist, each pitch of each diffractive gratingpattern being substantially the same, wherein only one diffractivegrating pattern having a convex portion or a concave portion of anoptical length is an odd multiple of half of one pitch of each of theother diffractive gratings; exposing a pattern having a plurality ofspace regions to the resist, the space region separating the diffractivegrating pattern; and transferring a resist pattern formed by developingthe resist to the semiconductor layer so as to form a first diffractivegrating region and a second diffractive grating region, wherein thefirst diffractive grating region and the second diffractive gratingregion have a plurality of segments combined to each other each of thesegments having a diffractive grating and a space region; and a segmentto which the only one diffractive grating pattern is transferred is oneof two segments of the first diffractive grating region or one of twosegments of the second diffractive grating region that are on theboundary side between the first diffractive grating region and thesecond diffractive grating region.
 2. The method as claimed in claim 1,wherein an interference exposure is used in the step of exposing thediffractive grating pattern to the resist.
 3. The method as claimed inclaim 2, wherein the resist is exposed with the diffractive gratingpattern through a phase-shifting mask having a step.
 4. The method asclaimed in claim 1, wherein an exposure mask having an openingcorresponding to the space region is used in the step of exposing thepattern having the space regions to the resist.